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An XQuery expression is evaluated in Query execution encompasses multiple steps:
Each of the steps allows for numerous optimizations, some of which Some rewritings are described in this article.
If the type Names of a value is known at compile time, type checks nodes can be removedspecified via name tests or predicates. In the example belowIf names are e.g. supplied via external variables, the static information that {{Code|$i}} will always reference items of type {{Code|xs:integer}} predicates can often be utilized to simplify the expressiondissolved:
for $i in 1 to 5return db:get('numberaddressbook')/descendant::city</syntaxhighlightpre>
Since {{Version|9.4}}, the * The last {{Code|for}} clause is merged into the {{Code|return}} clause and rewritten to a [[XQuery_3.0|#Simple_Map_Operator|simple map]] expression.
==Logic==(: rewritten to :)('a', '')[.]</pre>
Due to the compact syntax of XPath, it can make a big difference if a slash is added or omitted in a path expression. A classical example is the double slash {{Code|//}}, which is a shortcut for {{Code|descendant-or-node()/}}. If the query is evaluated without optimizations, all nodes of a document are gathered, and for each of them, the next step is evaluated. This leads to a potentially huge number of duplicate node tree traversals, most of which are redundant, as all duplicate nodes will be removed at the end anyway.==Database Statistics==
<syntaxhighlight lang="xquery">(: equivalent queries, with identical syntax trees :)doc('addressbook.xml')//city,doc('addressbook.xml')/descendant-or-node()/child::city;Count element nodes
(: rewritten to :)doc('addressbookThe number of elements that are found for a specific path need not be evaluated sequentially.xml')/descendant:Instead, the count can directly be retrieved from the database statistics:city</syntaxhighlight>
…unless the last step does not contain a positional predicate:<pre lang='xquery'>count(/mondial/country)
<syntaxhighlight lang="xquery">doc('addressbook.xml': rewritten to :)//city[1]231</syntaxhighlightpre>
As the positional test refers to the city child step, a rewritten query would yield different steps.;Return distinct values
Paths may contain predicates The distinct values for specific names and paths can also be fetched from the database metadata, provided that will be evaluated again by a later axis step. Such predicates are either shifted down or discardedthe number does not exceed the maximum number of distinct values (see {{Option|MAXCATS}} for more information):
a/b/c[d]</syntaxhighlight> Names of nodes can be specified via name tests or predicates. If names are e.g. supplied via external variables('Muslim', the predicates can often be dissolved: <syntaxhighlight lang="xquery">declare variable $name external := 'cityRoman Catholic';db:open(, 'addressbookAlbanian Orthodox', ...)<//*[name() = $name]pre>
(: rewritten to :)db:open('addressbook')//city</syntaxhighlight>==Index Rewritings==
=Index Rewritings=A major feature of BaseX is the ability to rewrite all kinds of query patterns for [[Indexes#Value Indexes|index access]].
A major feature of BaseX is the ability to rewrite all kinds of query patterns for [[Indexes|database index access]]. The following queries are all equivalent. They will all be rewritten to exactly the same query that will eventually access an the text indexof a <code>factbook.xml</code> database instance (the file included in our full distributions):
//name[text() = 'Shenzhen'],
=Changelog=;Version 9.6* Added: {{Option|UNROLLLIMIT}}
Text replacement - "<syntaxhighlight lang="xquery">" to "<pre lang='xquery'>"
This article is part of the [[XQuery|XQuery Portal]]. Optimizations are presented It presents some of the optimizations that speed up the execution time and reduce memory consumption. The article will be further extended with each version of BaseXqueries.
=Introduction=
# At parse time, the '''Parsing''': The query input string – an XQuery main module – is transformed to executable code. The result is a tree representation, called the ''abstract syntax tree'' (AST).# At compile time, the '''Compilation''': The syntax tree is decorated with additional information (type information, expression properties); expressions . Expressions (nodes) in the tree are relocated, simplified, or pre-evaluated. Logical optimizations are performed that do not rely on external information.# At evaluation time'''Optimization''': The dynamic context is incorporated: Referenced databases are opened and analyzed; queries are rewritten to use available indexes; accumulative and statistical operations (counts, summations, min/max, distinct values) are pre-evaluated; XPath expressions are simplified, based on the existence of steps. # '''Evaluation''': The resulting expression tree code is processedexecuted.# '''Printing''': The results are returned to the user. Some expression (such as simple loops) can be evaluated query result is serialized and presented in iterative mannera format that is either human-readable, whereas others (such as sort operations) need to or can be fully evaluated before the first result is availablefurther processed by an API.
If you run a query on [[Command-Line_Options#Standalone|command-line]], you can use {{Code|-V}} to output detailed query information. In the [[GUI]], you can enable the Info View panel.
=Compile-Time OptimizationsCompilation=
==Pre-Evaluation==
Parts of the query that are static and would be executed multiple times can already be evaluated at compile time:
<syntaxhighlight pre lang="'xquery"'>
for $i in 1 to 10
return 2 * 3
for $i in 1 to 10
return 6
</syntaxhighlightpre>
==Variable Inlining==
The value of a variable can be ''inlined'': The variables references are replaced by the expression that is bound to the variable. The resulting expression can often be simplified, and further optimizations can be triggered:
<syntaxhighlight pre lang="'xquery"'>
declare variable $INFO := true();
(: rewritten to :)
'Results: ' || count(//nodes)
</syntaxhighlightpre>
As the example shows, variable declarations might be located in the query prolog and in FLWOR expressions. They may also occur (and be inlined) in {{Code|try}}/{{Code|catch}}, {{Code|switch}} or {{Code|typeswitch}} expressions.
Functions can be inlined as well. The parameters are rewitten to {{Code|let}} clauses and the function is body is bound to the {{Code|return}} clause.
<syntaxhighlight pre lang="'xquery"'>
declare function local:inc($i) { $i + 1 };
for $n in 1 to 5
for $n in 1 to 5
return $n + 1
</syntaxhighlightpre>
Subsequent rewritings might result in query plans that differ a lot from the original query. As this might complicate debugging, you can disable function inling during development by setting {{Option|INLINELIMIT}} to {{Code|0}}.
==Static TypingLoop Unrolling== Loops with few iterations are ''unrolled'' by the XQuery compiler to enable further optimizations: <pre lang='xquery'>(1 to 2) ! (. * 2) (: rewritten to :)1 ! (. * 2), 2 ! (. * 2) (: further rewritten to :)1 * 2, 2 * 2 (: further rewritten to :)2, 4</pre> Folds are unrolled, too: <pre lang='xquery'>let $f := function($a, $b) { $a * $b }return fold-left(2 to 5, 1, $f) (: rewritten to :)let $f := function($a, $b) { $a * $b }return $f($f($f($f(1, 2), 3), 4), 5)</pre> The standard unroll limit is <code>5</code>. It can be adjusted with the {{Option|UNROLLLIMIT}} option, e.g. via a pragma: <pre lang='xquery'>(# db:unrolllimit 10 #) { for $i in 1 to 10 return db:get('db' || $i)//*[text() = 'abc']} (: rewritten to :)db:get('db1')//*[text() = 'abc'],db:get('db2')//*[text() = 'abc'],...db:get('db10')//*[text() = 'abc'],</pre> The last example indicates that index rewritings might be triggered by unrolling loops with paths on database nodes. The following expressions can be unrolled: * Simple map expressions* Simple FLWOR expressions* Filter expressions* [[Higher-Order Functions#fn:fold-left|fn:fold-left]], [[Higher-Order Functions#fn:fold-right|fn:fold-right]], {{Function|Higher-Order Functions|fn:fold-left1}} Care should be taken if a higher value is selected, as memory consumption and compile time will increase. ==Paths== Due to the compact syntax of XPath, it can make a big difference if a slash is added or omitted in a path expression. A classical example is the double slash {{Code|//}}, which is a shortcut for {{Code|descendant-or-node()/}}. If the query is evaluated without optimizations, all nodes of a document are gathered, and for each of them, the next step is evaluated. This leads to a potentially huge number of duplicate node tree traversals, most of which are redundant, as all duplicate nodes will be removed at the end anyway. In most cases, paths with a double slash can be rewritten to descendant steps… <pre lang='xquery'>(: equivalent queries, with identical syntax trees :)doc('addressbook.xml')//city,doc('addressbook.xml')/descendant-or-self::node()/child::city (: rewritten to :)doc('addressbook.xml')/descendant::city</pre> …unless the last step does not contain a positional predicate: <pre lang='xquery'>doc('addressbook.xml')//city[1]</pre> As the positional test refers to the city child step, a rewritten query would yield different steps. Paths may contain predicates that will be evaluated again by a later axis step. Such predicates are either shifted down or discarded: <pre lang='xquery'>(: equivalent query :)a[b]/b[c/d]/c (: rewritten to :)a/b/c[d]</pre>
<syntaxhighlight pre lang="'xquery"'>for declare variable $i in 1 to 5return typeswitch($i) case xsname external :numeric return = 'numbercity'; default return db:get('stringaddressbook')/descendant::*[name() = $name]
(: rewritten to :)
==FLWOR Rewritings==
* {{Code|where}} clauses are rewritten to predicates.
* {{Code|if}} expressions in the return clause are rewritten to {{Code|where}} clauses.
Various of these rewriting are demonstrated in the following example:
<syntaxhighlight pre lang="'xquery"'>for $a in 1 to 510
for $b in 2
where $a > 3
(: for is rewritten to let :)
for $a in 1 to 510
let $b := 2
where $a > 3
(: let is lifted up :)
let $b := 2
for $a in 1 to 510
where $a > 3
let $c := $a + $b
(: the where expression is rewritten to a predicate :)
let $b := 2
for $a in (1 to 510)[. > 3]
let $c := $a + $b
return $c
(: $b is inlined :)
for $a in (1 to 510)[. > 3]
let $c := $a + 2
return $c
(: $c is inlined :)
for $a in (1 to 510)[. > 3]
return $a + 2
(: the remaining clauses are merged and rewritten to a simple map :)
(1 to 510)[. > 3] ! (. + 2)</syntaxhighlightpre> ==Static Typing== If the type of a value is known at compile time, type checks can be removed. In the example below, the static information that {{Code|$i}} will always reference items of type {{Code|xs:integer}} can be utilized to simplify the expression: <pre lang='xquery'>for $i in 1 to 5return typeswitch($i) case xs:numeric return 'number' default return 'string' (: rewritten to :)for $i in 1 to 5return 'number'</pre> ==Pure Logic== If expressions can often be simplified: <pre lang='xquery'>for $a in ('a', '')return $a[boolean(if(.) then true() else false())] (: rewritten to :)for $a in ('a', '')return $a[boolean(.)] (: rewritten to :)for $a in ('a', '')return $a[.]
Boolean algebra (and set theory) comes with a set of laws that can all be applied to XQuery expressions.
| Distributivity
|- valign="top"
| <code>$a and or not($a)</code>
| <code>true()</code>
| Tertium non datur
* <code>true#0 and true#0</code> must raise an error; it cannot be simplified to <code>true#0</code>
=Optimization=Paths== Some physical optimizations are also presented in the article on [[Indexes|index structures]].
In most caseseach database, paths with a double slash metadata is stored that can be rewritten utilized by the query optimizer to descendant steps…speed up or even skip query evaluation:
<syntaxhighlight pre lang="'xquery"'>distinct-values(: equivalent query :)a[b]/b[c/d]/creligions)
(: rewritten to :)
<syntaxhighlight pre lang="'xquery"'>declare context item := db:openget('factbook');declare variable $DB := 'factbook';
//name[. = 'Shenzhen'],
//name[data() = 'Shenzhen'],
//name[./text() = 'Shenzhen'],
//name[text()[. = 'Shenzhen']],
//name[string() = 'Shenzhen'],
//name[string() = 'Shen' || 'zhen'],
//name[./data(text()/string()) = 'Shenzhen'],//name[text() ! data() ! string() = 'IcelandShenzhen'], //name[. eq 'Shenzhen'],//name[not(. ne 'Shenzhen')],//name[not(. != 'Shenzhen')],.//name[. = 'Shenzhen'],//*[local-name() = 'name'][data() = 'Shenzhen'],
db:openget('factbook')//name[. = 'Shenzhen'],db:get($DB)//name[. = 'Shenzhen'],
for $name in //name[text() = 'Shenzhen']
(: rewritten to :)
db:text('factbook', 'Shenzhen')/parent::name
</syntaxhighlightpre> Multiple element names and query strings can be supplied in a path: <pre lang='xquery'>//*[(ethnicgroups, religions)/text() = ('Jewish', 'Muslim')] (: rewritten to :)db:text('factbook', ('Jewish', 'Muslim'))/(parent::*:ethnicgroups | parent::*:religions)/parent::*</pre> If multiple candidates for index access are found, the database statistics (if available) are consulted to choose the cheapest candidate: <pre lang='xquery'>/mondial/country [religions = 'Muslim'] (: yields 77 results :) [ethnicgroups = 'Greeks'] (: yields 2 results :) (: rewritten to :)db:text('factbook', 'Greeks')/parent::ethnicgroups/parent::country[religions = 'Muslim']</pre> If index access is possible within more complex FLWOR expressions, only the paths will be rewritten:
<pre lang='xquery'>for $country in //countrywhere $country/ethnicgroups = 'German'order by $country/name[1]return element { replace($country/@name, ' ', '') } {}, (: rewritten to :)for $country in db:text('factbook', 'German')/parent::ethnicgroups/parent::countryorder by $country/name[1]return element { replace($country/@name, ' ', '') } {}</pre> The [https://projects.cwi.nl/xmark/ XMark XML Benchmark] comes with sample auction data and a bunch of queries, some of which are suitable for index rewritings: ;XMark Query 1 <pre lang='xquery'>let $auction := doc('xmark')return for $b in $auction/site/people/person[@id = 'person0']return $b/name/text() (: rewritten to :)db:attribute('xmark', 'person0')/self::attribute(id)/parent::person/name/text()</pre> ;XMark Query 8 <pre lang='xquery'>let $auction := doc('xmark')return for $p in $auction/site/people/person let $a := for $t in $auction/site/closed_auctions/closed_auction where $t/buyer/@person = $p/@id return $t return <item person="{ $p/name/text() }">{ count($a) }</item>, (: rewritten to :)db:get('xmark')/site/people/person ! <item person='{ name/text() }'>{ count( db:attribute('xmark', @id)/self::attribute(person)/parent::buyer/parent::closed_auction )}</item></pre> If the accessed database is not known at compile time, or if you want to give a predicate preference to another one, you can [[Indexes#Enforce Rewritings|enforce index rewritings]]. =Evaluation-Time Optimizations=
==Comparisons==
In many cases, the amount of data to be processed is only known after the query has been compiled. Moreover, the data that is looped through expressions may change. In those cases, the best optimizations needs to be chosen at runtime.
If sequences of items are compared against each other, a dynamic hash index will be generated, and the total number of comparisons can be significantly reduced. In the following example, <code>count($input1) * count($input2)</code> comparisons would need to be made without the intermediate index structure:
<syntaxhighlight pre lang="'xquery"'>
let $input1 := file:read-text-lines('huge1.txt')
let $input2 := file:read-text-lines('huge2.txt')
return $input1[not(. = $input2)]
</syntaxhighlightpre>
==Pre-Evaluation=Changelog=
Introduced with Version 9.4.
